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Network Working Group Jonathan B. Postel RFC # 516 UCLA-NMC NIC # 16683 May 18, 1973

                       LOST MESSAGE DETECTION

I have three suggestions for detecting the loss of messages by the communications subsystem. The first of these is perhaps the more powerful and simpler to implement since it uses no new concepts and has the power to retransmit the message detected as lost.

The first scheme:

  If upon sending a message the host saved a copy of that message and
  waited until either:
      a RFNM was returned, in which case everything is ok and the next
      message is processed.
      a INCOMPLETE TRANSMISSION is returned, in which case the copy of
      the message is retransmitted (this could be a loop so put a
      finite upper bound on the number of times to retransmit the same
      a DESTINATION DEAD is returned, in which case mark the
      destination down and require the exchange of reset commands
      before further communication is allowed.
      something else is received indicating an error in the network or
      local IMP, in which case at least log the error, and probably
      close the conversation.
  Following the above procedures either on a per host basis or a per
  link basis should prevent a lost message problem from

The second scheme:

  If on a per host basis, message numbers are included in the host to
  host header of messages,, and messages are delivered in order (this
  is currently the case in the network, except for priority messages
  so this proposal requires that each host either send everything as
  priority or nothing as priority) then each receiving host can detect
  a missing message by comparing the message number of the received
  message with the previously received message.
      On exchanging resets the sequence numbers between that pair of

Postel [Page 1] RFC 516 LOST MESSAGE DETECTION May 1973

      hosts is set to zero.
      Each time a message is sent the current send message number is
      entered into a field in the message header, and the current send
      message number is incremented (modulo N, say N=256)
      Each time a message is received the message number from the
      message is compared to the current receive message number and:
          if the received message is the expected one then the message
          is acceptable and current receive message number is
          incremented (modulo N);
          if the received message is not the expected one then a
          message has been lost.
  What to do when a missing message is detected, not clear, but at
  least can be logged and reported to the network control center.  A
  missing message may not be fatal to an interactive conversation, but
  it is critical in a file transfer, thus I suggest that missing
  messages which are not recovered be cause to close the conversation.

The third scheme:

  Host to host acknowledgements could be required.  Such an
  acknowledgement scheme could be implemented similarly to the IMP to
  IMP scheme.  This is a serious change to the current protocols so I
  will not elaborate on it here, feeling that deeper study will be
  necessary to fully specify a reasonable host to host acknowledgement

Of these three suggestions the first is the most immediately practical and implementable; in fact several hosts all ready do this. These schemes also are non-conflicting, they could be implemented and used simultaneously.

     [ This RFC was put into machine readable form for entry ]
     [ into the online RFC archives by Alex McKenzie with    ]
     [ support from GTE, formerly BBN Corp.             9/99 ]

Postel [Page 2]

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